Cancer predisposition syndromes are autosomal dominantly inherited conditions in which the affected individuals have a high relative risk for developing cancer. PTEN and neurofibromatosis-2 (NF2) are tumor suppressors that are inactivated in both somatic cancers and in cancer predisposition syndromes that belong to a clinically defined group (phakomatoses). The mechanism for tumor suppression is known only for PTEN and relies on its ability to dephosphorylate phosphoinositides and antagonize the growth promoting effects of the phosphatidylinositol-3 OH (PI-3) kinase. The regulation of both PTEN and NF2 (merlin) is not yet understood, but it appears to depend on the recruitment of both proteins to the plasma membrane and on their phosphorylation. In spite of the similarities between these tumor suppressors, no previous study attempted to link them in a common pathway. The broad objective of this project is to characterize a molecular connection between PTEN and NF2 tumor suppressors. The new finding that PTEN associates with an adaptor molecule that was previously shown to bind to NF2 will constitute the major focus of this study. The detailed project attempts to cover the following topics: (1) the role of the adaptor molecule in cell growth and migration of cultured gene-deficient cells, and in tumor formation and metastasis in the gene-deficient animals already generated in the laboratory; (2) the analysis of the complex between the three molecules in terms of specific binding affinities, size of the complex by gel filtration and membrane co-localization by density gradient fractionation and immunofluorescence; and (3) interrelations in regulation and signaling between PTEN and NF2 in terms of phosphorylation, stability, Akt/PKB kinase and Rac GTP-ase activation. Finding a common molecular link between PTEN and NF2 tumor suppressors that determine similar cancer predisposition syndromes and are frequently inactivated in brain cancer is fundamental to the understanding of the pathogenesis of cancer and constitutes the basis of a targeted cancer therapy.